I've answered to the support thread opened on the same subject. Here is a quote of the answer:
The NumericTextBox widget renders two input elements - one for edit value and one for the formatted value. If you would like to change the text color of the widget, then you will need to do this for both input elements. Check this demo that demonstrates how to accomplish this task.
I would like to ask you to continue our discussion there if needed to avoid any duplication.
Join us on our journey to create the world's most complete HTML 5 UI Framework - download Kendo UI